Polarization splitters are optical elements which enable light to be broken down into its different polarization components. The direction of polarization of the light is defined with respect to the oscillation plane of the electric field. More often than not, non-polarized light is broken down into its two orthogonal linear polarizations. This being the case, a distinction is made between (perpendicular) S polarization and (parallel) P polarization. In S polarized light, the oscillation plane is perpendicular to the plane of incidence defined by the normal line of the surface and the incidence vector. In P polarization light, the oscillation plane is parallel to the plane of incidence. The components can be split by absorption or by reflection.
Generally speaking, polarization splitters transmit P polarization and reflect S polarization. It is generally accepted that an ideal polarization splitter reflects all the polarized light perpendicularly to the plane of incidence (S), whereas it transmits all the polarized light parallel with the plane of incidence (P) (for a given wavelength). The efficiency of the polarization splitting function may be expressed as the product of the spectral reflection of the S polarization (Rs) multiplied by the spectral transmission of the P polarization (Tp), namely (Rs)×(Tp) (at a given wavelength). It is also generally acknowledged that the aim in producing a polarization splitter is to achieve an efficiency in excess of 80% and preferably in excess of 90%.
Polarization splitters lend themselves to various applications, which include ophthalmic lenses incorporating inserts for projecting an image towards a user.
By this is meant ophthalmic lenses of image-combining systems for spectacles or masks; an image is projected towards the eye of the wearer via an optical path determined by the lens; the term “lens” is then used to refer to the optical system containing the inserts, which may be designed in particular to be mounted in a spectacle frame or in a mask. The inserts may contain mirrors, beam splitters, polarization splitter cubes, quarter-wave plates, lenses, mirrors, concave reflective lenses (a Mangin mirror, for example), diffraction lenses and/or holographic components. A device for projecting images towards the user will then comprise the lens mounted in spectacles or masks and an image source such as a micro-screen, for example a liquid crystal micro-screen, more specifically a Kobin CyberDisplay 320 micro-display.
In such applications, the polarization splitter elements are used to process the polarized light emitted by the micro-screens currently used, such as micro-displays.
One example of such an ophthalmic lens is illustrated in FIG. 1. The image is emitted by a source 1. The source 1 may be a miniaturized micro-screen such as a liquid crystal micro-display emitting polarized light (P). The optical system of the projecting ophthalmic lens 10 comprises a field lens 2. A mirror 3 and the polarization splitter 4 are placed so as to intercept the optical path travelled by the image inside the ophthalmic lens 10. Bonded to the polarization splitter 4 is a quarter-wave plate 5 and a Mangin mirror 6.
The ophthalmic lens 10 operates in the following manner. Polarized light from the source 1 passes firstly through a field lens 2. Having passed through it, it is reflected by a mirror 3, which returns turns it through an angle of 90°. The light then passes through the polarization splitter 4, whereby one of the polarization components (S) is reflected and the other (P) is transmitted. The transmitted component passes through a quarter-wave plate 5, the axes of which are arranged at 45° relative to the propagation direction P in the plane perpendicular to the propagation direction, then strikes a Mangin mirror 6, which reflects the light so that it is sent back through the quarter-wave plate. The light, which is now S polarized, is reflected by the polarization splitter towards the eye of the observer. Consequently, this embodiment enables the polarized light transmitted by the micro-screen to be sent back towards the eye 7.
However, a device of this type incorporating an “ideal” polarization splitter has a drawback in that, in terms of ophthalmic function, it directs only 50% of the light from an object towards the eye because 50% of this light is S polarized and is therefore reflected by the splitter.
The following definitions will be used throughout this description.
See-through image: see-through image refers to the image of a scene as viewed when the light rays pass directly through the polarization splitter element.
Screen image: screen image refers to the image of a light source (in our example a micro-screen) passing though the lens inserted in the display glass, as illustrated in FIG. 1.
Efficiency of the polarization splitting function: see above.
Efficiency of the vision see-through function: this refers to the value of the mathematical integral of the product in terms of non-polarized light of the polarization splitter [=½*(Tp+Ts) multiplied by the emission spectrum of the source divided by the integral of the emission spectrum of the source [in the spectral domain in question].
Transmission of the imaging function of the display glass: this refers to the value of the mathematical integral of the product of the spectral transmission of the imaging function of the display glass (defined by the optical path in FIG. 1) multiplied by the emission spectrum of the source divided by the integral of the emission spectrum of the source [in the spectral domain in question].
It should be noted that transmission of the imaging function of the display glass as well as transmission of the see-through vision function may also be weighted by the spectral sensitivity, of the eye. This is then referred to as “photopic transmission of the imaging function of the display glass”. The normalized curve Y of the CIE (Commission International de l'Eclairage) 2° observer will be used as the curve representing the spectral sensitivity of the eye.
Locally centered: a curve is said to be locally centered on the emission spectrum of the light source when:                the spectrum of the emission source assumes the form of peaks (see FIG. 2, for an example of this);        around an emission peak of the light source, within a restricted spectral domain with an order of magnitude that is twice the mid-height width of the emission peak of the source, the curve follows a peak (or a valley), the local extremum of which is not spectrally distant from the apex of the emission peak of the source by greater than the value of the mid-height width of this peak;        alternatively, the Rayleigh criterion may also be applied, on the assumption that two curves are locally centered.        